Radar plotting and collision avoidance

From radar picture to radar plot

Lesson 22 covered conduct in fog (Rule 19), lesson 33 AIS with its CPA and TCPA, and lesson 34 the basics of radar. A radar picture shows where each echo is now; radar plotting turns two or more of those positions into what you need to know: how close a vessel will come, when, what she is really doing, and what to do about it.

Why plot when AIS does it for you? Many vessels carry no AIS, or have it switched off, and AIS data can be wrong; radar shows anything that returns a strong enough echo. And Rule 7 expects it: proper use of radar, if fitted and working, includes long-range scanning for early warning and radar plotting, or equivalent systematic observation, of the echoes you detect.

Every example in this lesson starts from one invented situation on the fictional practice chart (the vessels are invented too). At 1400 one afternoon, fog has closed in south of the Isle of Kerran. You are at 49°40.9'N 004°08.4'W, motoring 090°T at 5 kn to pass south of South Brennan buoy, sounding one prolonged blast at intervals of not more than two minutes. It is near slack water, so take the tidal stream as nil. The radar, on its 6 M range, shows three echoes:

Reading the display

The radar picture is a plan of the sea with your boat at its centre. The heading marker is a line from the centre to the edge showing where your bow points; the range rings, VRM and EBL measure each echo (lesson 34). For plotting, which way up the picture is drawn matters:

A yacht in a seaway may yaw ten degrees or more either side of her course. Head-up draws each sweep relative to wherever the bow happened to point, so a steady echo is painted in a slightly different place each time and smears into an arc, and its bearing is uncertain by as much as the yaw. A heading sensor (an electronic compass connected to the radar) lets the set correct each sweep: that is what north-up and course-up do, giving a still picture and true bearings ready to plot.

With head-up you can still plot, but convert each bearing: true bearing = relative bearing + heading (subtract 360 if the answer passes 360), using the heading at the moment you read the EBL — have the helmsman call it. At 1400 the ship’s echo is 021° relative with the boat’s head on 090°T: 021° + 090° = 111°T.

Relative and true motion

Most small-craft radars show relative motion: you stay at the centre and every echo moves relative to you. A buoy slides past opposite to your course, at your speed; another vessel’s echo moves along her relative track, the combination of her motion and yours — which shows how close she will come. A true-motion display, offered by some sets, moves your own boat across the screen instead, so the coast stays (more or less) still and each vessel moves along her true track. To do that the set needs your heading, from a heading sensor, and your speed, from the log or GPS; it works only with a stabilised (north-up or course-up) picture. Some sets can leave a fading echo trail behind each echo: in relative motion it traces the relative track, in true motion the true one.

What a small-craft radar can miss

Rule 6 asks every vessel using radar to allow for its limitations when deciding a safe speed. On a yacht they are considerable.

• Scanner height and range. Radar waves bend a little more than light, so the radar horizon is roughly 2.2 × √(height in metres) miles, against 2.08 × √(height of eye) for the eye (lesson 34) — a rough guide. A scanner 5 m up sees about 4.9 M to the horizon, and a small boat 2 m high adds 3.1 M: 8.0 M in theory, but her echo is so weak you may not see her until she is much closer.
• Sea clutter. Waves near the boat return echoes that fill the middle of the screen, more in rough seas and usually more to windward. The sea clutter control reduces it, but turned up too far it removes real echoes too.
• Rain clutter. Rain and showers show as fuzzy patches that can hide a vessel inside them, and heavy rain can weaken echoes from beyond it. The rain control helps, with the same caution.
• Shadow sectors. The mast, rigging and other gear near the scanner can block the beam in some directions, leaving blind or shadow sectors; a ship or headland also hides whatever is behind it. Find your own boat’s shadow sectors in clear weather. On a sailing yacht, heeling tilts the beam as well, which can weaken echoes when she is well heeled.
• Small or low targets. Yachts, small fishing boats, RIBs and buoys without reflectors give weak echoes that come and go. A small scanner’s wide beam smears echoes sideways, so bearings are less precise than ranges: put the EBL through the middle of the echo.

Plotting a target: the O, A, W triangle

Take two observations of the echo a few minutes apart — six is convenient, because six minutes is a tenth of an hour — while you hold a steady course and speed. Plot them on a plotting sheet (a north-up polar diagram, like the one below), or straight onto a north-up screen if your set lets you mark positions.

1. O: plot the first observation and label it with the time.
2. A: plot the second, and label it.
3. Relative track: join O to A and extend the line past the centre. As long as neither vessel alters, the echo moves along it.
4. CPA: drop a perpendicular from the centre to the relative track. Its length is the closest point of approach; its direction from the centre is the bearing she will be on at that moment.
5. TCPA: O→A in the interval gives the relative speed; the distance from A to the CPA divided by the relative speed gives the time.
6. W: from O, lay off the reciprocal of your own course for the distance you ran through the water in the interval.
7. W→A: her true course and speed (through the water). The angle between that and the line from her to you is her aspect.

W to O is your own way; O to A is her relative movement; W to A is her true movement.

Why does it work? If she were stopped, her echo would move exactly opposite to your course at your speed — from O to W. Wherever she actually got to, A, the difference between the two is her own movement: W to A.

Two plots give an estimate, not a certainty. Radar bearings on a yacht can easily be a degree or two out, and a small CPA is very sensitive to them. Keep plotting every few minutes: if the plots stay on a straight line, the prediction holds; if they curve away, she is altering course or speed, and you start again from a fresh O.

Is there a risk of collision?

Rule 7 (lesson 20): use all available means; if in doubt, assume risk exists. If the compass bearing of an approaching vessel does not appreciably change, risk exists — but risk may exist even with an appreciable change, especially with a very large vessel, a tow, or at close range. And make no assumptions on scanty information, especially scanty radar information.

The ship shows why the plot matters. Her bearing changed by 2° in six minutes, drawing slowly ahead — the kind of change it is tempting to call “appreciable”. The plot says she will cross 0.56 M ahead of you.

The rules give no distance for a close-quarters situation, so decide the CPA you want before you need it. Here the skipper wants at least 1 M from any vessel in fog — a choice, not a rule; you might want more from a fast ship or with an uncertain plot. On that standard:

• The ship (0.56 M at 1423): a close-quarters situation is developing. Act.
• The fishing vessel (1.46 M at 1425, steering about 190°T at 4.0 kn): passes clear for now, but vessels fishing alter course and speed often. Keep plotting her.
• The buoy: stationary, as charted. No risk.

Choosing a manoeuvre

When vessels cannot see each other there is no give-way or stand-on vessel: each must take her own avoiding action. Rule 19(d) (lesson 22) says that a vessel which detects another by radar alone must decide whether a close-quarters situation is developing or a risk of collision exists, and if so take avoiding action in ample time. If that action is an alteration of course, then so far as possible avoid:

• an alteration to port for a vessel forward of the beam, other than one being overtaken; and
• an alteration towards a vessel abeam or abaft the beam.

Rule 8 adds that the action should be large enough to be readily apparent to another vessel watching by radar — avoid a succession of small alterations — and that you check it is working until she is finally past and clear. Slowing down, or stopping, is avoiding action too. And Rule 19(e) still applies: if you cannot avoid a close-quarters situation with a vessel forward of the beam, reduce speed to the minimum at which you can be kept on course, and take all way off if necessary.

To test a manoeuvre on the plot, keep W and A where they are (her motion, W→A, doesn’t change). From W lay off your new course and speed for the same interval, to a new point O′. Her new relative motion is O′→A: draw a line parallel to it through her position when you alter (A, if you alter at once) and measure the new CPA.

After the alteration

Make the alteration, then keep plotting. Within a few minutes her echo should be moving along the new relative track. If it isn’t, she has altered as well: start a fresh plot from her latest position and think again. Keep watching until she is finally past and clear — past her CPA with the range opening — then come back to your course. Fog signals keep going all the while.

If she comes into sight, the situation changes. Rule 19 is for vessels not in sight of one another; once you can see each other, the steering and sailing rules for vessels in sight of one another (Rules 11 to 18, lesson 20) apply, with their give-way and stand-on vessels.